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Abstract

A novel technique, designated dual imaging and modeling evaluation (DIME), for evaluating single-laser shot fluorescence lifetimes is presented. The technique is experimentally verified in a generic gas mixing experiment to provide a clear demonstration of the rapidness and sensitivity of the detector scheme. Single-laser shot fluorescence lifetimes of roughly 800 ps with a standard deviation of ~120 ps were determined. These results were compared to streak camera measurements. Furthermore, a general fluorescence lifetime determination algorithm is proposed. The evaluation algorithm has an analytic, linear relationship between the fluorescence lifetime and detector signal ratio. In combination with the DIME detector scheme, it is a faster, more accurate and more sensitive approach for rapid fluorescence lifetime imaging than previously proposed techniques. Monte Carlo simulations were conducted to analyze the sensitivity of the detector scheme as well as to compare the proposed evaluation algorithm to previously presented rapid lifetime determination algorithms.

Figures (6)

Schematic illustration of the experimental setup. The laser beam is expanded using a spherical telescope (ST) and then focused to a laser sheet in the measurement volume with a cylindrical lens (CL). A trig pulse (TP) is sent to the two ICCD cameras and to a trigger box (TB) which triggers both the streak camera and the MCP-PMT. A 70/30 beam splitter (BS) is located in the front of the camera lenses.

PLIF images and graphical illustrations of signal simulations. Simultaneous, single-laser shot PLIF images of a toluene-seeded jet in a nitrogen co-flow are seen in (aexp) and (bexp) detected with a 2 ns and a 400 ns gated camera, respectively. In (asim) and (bsim), graphical descriptions of simulations of ICCD-camera signal detection are displayed. The red curves are simulated LIF signals with lifetimes of 7 ns, the blue curve in (asim) is the 2 ns camera gain function while the rising flank of the 400 ns gain function is seen in (bsim). The gray areas are the simulated signals detected by the two ICCD cameras, using Eq. (1).

Fluorescence lifetimes evaluated from 900 streak-camera accumulations (dashed and solid lines) as well as from single shot FLI detector images (filled and open circles with error bars). Two mixtures of oxygen and nitrogen were used as ambient quenching molecules; 10.5/89.5 (open circles and dashed line) and 17/83 (filled circles and solid line).

The signal-to-noise propagation of the system is illustrated with the figure of merit (Eq. (5)). The experiment was modeled by Monte Carlo simulations for two different delay times for the 2 ns camera gain curve. The solid line corresponds to the settings that were used in the experiments, whereas the dashed line illustrates the figure of merit when the 2 ns camera gain function is advanced 0.5 ns in time. Evidently, the sensitivity of the technique is improved if the gain curve is temporally advanced but, on the other hand, less photons are detected, resulting in a degradation of the signal to nose ratio. The red and blue crosses are the experimental F-values corresponding to the measurement presented in Fig. 4 (the same color coding has been used).

Monte Carlo simulations of FLI with a mean value of 350 detected photons were performed for three different sets of gain functions. (a) The blue curve is a fluorescence curve with a lifetime of 8 ns. Detection using two square gain curves is seen in the upper plot. Two different approaches were tested; standard rapid lifetime determination (SRLD) and optimized rapid lifetime determination, proposed by Chan et al. [30]. For SRLD Δt is 3 ns, Y and P are 1 and T is 6 ns, meaning that we have two gain functions with equal width where the first one closes as the other opens. For ORLD Δt is 3 ns, Y is 0.25, P is 12 and T is 36 ns. In the lower plot, two ramped gain curves are used which are described by Eqs. (7a) and (7b) (the constant B is set to 40 ns). (b) The figure of merit corresponding to the simulated results using ramped gain curves is represented by the black curve. (c) The error of the mean value of the determined fluorescence lifetime. The SRLD as well as ORLD are unable to predict short lifetimes since the signal enhanced by the latter of the two gain functions (dashed red curve) is very weak. For longer lifetimes, the SRLD breaks down. (d) The SNR for the detected fluorescence lifetime is nearly constant for the ramped-gain curve configuration. The square gain configurations have lifetime dependences on their SNR with clear optima.